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UNIT – III
II: Multiplexing & T-Carriers
MULTIPLEXING:
Multiplexing is the transmission of information from more than one source to
more than one destination over the same transmission medium (facility).
Although transmissions occur on the same facility, they do not necessarily
occur at the same time.
The transmission medium may be a metallic wire pair, a coaxial cable, or a
satellite microwave system.
There are many domains in which multiplexing can be accomplished including
space, time, frequency and wavelength.
The most predominant methods are time-division multiplexing (TDM),
frequency-division multiplexing (FDM), wavelength-division multiplexing (WDM).
TIME DIVISION MULTIPLEXING
TDM allows transmission from multiple sources on the same facility but not at
the same time.
Transmissions from various sources are interleaved in the time domain.
PCM is the most common type of modulation used with TDM (PCM-TDM
system).In TDM, each signal occupies a subset of the transmission facility
bandwidth for a slice of time (time slot)
Figure 6.1 Dividing a link into channels
Figure 6.2 Categories of multiplexing
Figure 6.12 TDM
Note
TDM is a digital multiplexing technique
for combining several low-rate
channels into one high-rate one.
Figure 6.13 Synchronous time-division multiplexing
Note
In synchronous TDM, the data rate
of the link is n times faster, and the unit
duration is n times shorter.
Figure 6.15 Interleaving
Figure 6.18 Empty slots
Figure 6.19 Multilevel multiplexing
Figure 6.20 Multiple-slot multiplexing
Figure 6.21 Pulse stuffing
Figure 6.22 Framing bits
Figure 6.26 TDM slot comparison
DIGITAL SIGNAL – LEVEL 0 (DS-0)
The fundamental building block for any TDM system is DS-0.
The DS-0 channel occupies a 64 kbps bandwidth
8000 sam ples 8 bits
x
 64 Kbps
sec ond
sam ple
1 CHANNEL SYSTEM  OCCUPIES 64 Kbps BANDWITH
2-CHANNEL SYSTEM  OCCUPIES 128 Kbps BANDWITH
--------n-CHANNEL SYSTEM  OCCUPIES n*64 Kbps BANDWITH
•In this modulation technique, an analog signal is digitized, and interleaved
with other digitized voice signal to create a single bit stream.
•At the receiving end, the bit stream is decomposed into separate digital
streams of lower frequencies, each stream is then converted back into what
resembles the original voice signal.
CHANNEL PCM-TDM SYSTEM (a)block diagram (b)TDM frame
The sampling rate used for voice = 8000 samples/sec
Therefore, Sampling Interval = 1/8000 = 125µs
•This means that the time between two consecutive samples (from the same source) is
125µs.
•TDM systems exploit this fact and utilize this interval to sample signals from other
subscribers. In T1 systems the signals from 24 subscribers is sampled in 125µs.
•The samples are quantized and then converted into a bit-stream for transmission over
the channel.
•The PCM code for each channel occupies a fixed time slot (epoch) within the total TDM
frame0
T1 Digital Carrier System
•The T1 carrier system multiplexes binary code-words corresponding to samples
of each of the 24 channels in a sequence.
•A segment containing one codeword (corresponding to one sample) from each
of the 24 channels is called a FRAME.
Each frame has 24x8 = 192 data bits and takes 125µs.
•As mentioned previously, sampling rate used for voice = 8000 samples/sec
Every sample is represented by 8 bits
Therefore,
Data rate of 1 voice channel = 8x8000 = 64kbps
•In the T1 system 24 voice channels are multiplexed in time
Data rate of a T1 stream should be = 24x64kbps = 1.536 Mbps
•At the receiver it is also necessary to know where a frame starts in order to
separate information bits correctly. For this purpose, a Framing bit is added at
the beginning of each frame.
Framing Bits: Indicate start of frames.
Total number of bits/ frame = 193
the actual data rate = 1.544Mbps
D-type channel banks
•Early TDM systems used D1 digital channel banks (PCM encoders) with a
seven bit sign magnitude only PCM code, analog companding, and a µ=100.
•The framing bit sequence was simply a 1/0 pattern with the early digital channel
banks.
•Later version digital channel banks (D2 and D3) added an eight bit called the
signaling bit to each PCM code for the purpose of interoffice signaling (On-Hook/
off-Hook signals, Alarm signals).
•Modern versions use digitally companded, eight-bit sign magnitude compressed
PCM codes, with a µ=100
Superframe TDM format
Each frame has 24x8 = 192 data bits and takes 125µs.
Total number of bits/ frame = 193
The actual data rate = 1.544Mbps
Composite frame alignment
Frame1(125µs)
1
Frame2(125µs)
0
Frame3(125µs)
0
Frame4(125µs)
0
Frame5(125µs)
1
Frame6(125µs)
1
Frame7(125µs)
0
Frame8(125µs)
1
Frame9(125µs)
1
Frame10(125µs)
1
Frame11(125µs)
0
Frame12(125µs)
0
•Framing bit sequence for the T1 super frame format using D2 and D3 channel
banks
DS-1
EXTENDED SUPERFRAME (ESF)
•Another framing format recently developed for new designs of T1 carrier
systems is the extended super frame format. It consists of 24 frames.
•Each frame consists of 24 DS-0 64 kbps channels
•Each frame has a framing bit (f-bit)
•Each frame consists of 193 bits
•The ESF consists of 193 x 24 = 4632 bits of which 24 are framing bits
4632bits/1.544mbps = 3ms
However only 6 out of the 24 f-bits are used for synchronization (frames: 4, 8, 12,
16, 20 and 24)
Extended Superframe
1
1
0
1
0
0
f f f
f f f
f f f
f f f
f f f
f f f
f
f
f
f
f
f
12
4
16
8
20
24
Figure 2.40: Frame Pattern Sequence in the T1
extended superframe format.
•Framing bit sequence = 0 0 1 0 1 1
•Another 6 of the framing bits are used for cyclic check redundancy (CRC-6).
•The CRC-6 bits occur in frames: 1, 5, 9, 13, 17, 21
•CRC-6 is used for error detection
•The 12 remaining framing bits occur in frames:
2, 3, 6, 7, 10, 11, 14, 15, 18, 19, 22, 23
•These 12 framing bits provide for a management channel called the facilities
data link (FDL).
•In ESF: Signaling bit in 6th frame
Signaling bit in 12th frame
Signaling bit in 18th frame
Signaling bit in 24th frame
 a-bit
 b-bit
 c-bit
 d-bit
These signaling bit streams are sometimes called the a,b,c and d signaling channels or
signaling highways
EXTENDED SUPERFRAME FORMAT
FRAME Number
F-BIT
FRAME Number
F-BIT
1
CRC-6
13
CRC-6
2
FDL
14
FDL
3
FDL
15
FDL
4
S=0
16
S=0
5
CRC-6
17
CRC-6
6
FDL
18
FDL
7
FDL
19
FDL
8
S=0
20
S=1
9
CRC-6
21
CRC-6
10
FDL
22
FDL
11
FDL
23
FDL
12
S=1
24
S=1
Figure 6.25 T-1 frame structure
Table 6.2 E line rates
Figure 6.23 Digital hierarchy
Table 6.1 DS and T line rates
Figure 6.24 T-1 line for multiplexing telephone lines
Fractional T Carrier Service
1.544Mbps
(Users 1,2,3,and 4)
•T1 carriers provide a higher bit rate than most users require.
•Fractional T1 systems distribute the channel in a standard T1 system among
more than one user, allowing several subscribers to share one T1 line.
•Bit rates offered are 64 kbps (1 channel), 128 kbps (2 channels), 256 kbps (4
channels), 384 kbps (6 channels), 512 kbps (8 channels) 768 kbps (12 channels)
being the most common.
•The minimum data rate necessary to propagate video information is 384 kbps.
•The data service unit/channel service unit (DSU/CSU) is a digital interface that
provides the physical connection to a digital carrier network.
•User 1 is allocated 128 kbps, 256 kbps for user 2, 384 kbps for user 3, and 768
kbps for user 4 for a total of 1.536 kbps (8 kbps is reserved for the framing bit).
North American Digital Multiplexing Hierarchy
•The American Telephone and Telegraph Company’s (AT&T’s) North American
Digital Hierarchy for multiplexing digital signals into a single higher-speed pulse
stream suitable for transmission on the next higher level of the hierarchy.
•To upgrade from one level in the hierarchy to the next higher level, a special
device called muldem (multiplexer/ demultiplexer) is required.
•Muldems can handle bit-rate conversions in both directions. The muldem
designations (M12, M23, and so on) identify the input and output digital signals
associated with that muldem.
•The DS-1 may be further multiplexed or line encoded and placed on specially
conditioned cables called T1 lines.
•The DS-2, DS-3, DS-4 and DS-5 digital signals are placed on T2, T3, T4M, or
T5 lines, respectively.
•Digital signals are routed at central locations called digital cross-connects.
•A digital cross-connect (DSX) provides a convenient place to make patchable
interconnects and to perform routine maintenance and troubleshooting.
•Each type of digital signals (DS-1, DS-2, and so on) has its own digital switch
(DSX-1, DSX-2, and so on).
•The output from a digital switch may be upgraded to the next higher level of
multiplexing or line encoded and placed on its respective T lines (T1, T2, and so
on).
Digital Line Encoding
Digital line encoding involves converting standard logic levels(TTL,CMOS) to a form
more suitable to telephone line transmission. Essentially six primary constants are
considering when selecting line encoding format
1) Transmission voltages and dc component
2) Duty cycle
3) Bandwidth considerations
4) Clock and framing bit recovery
5) Error detection & Ease of detection and decoding
6) Digital biphase.
Transmission voltages and dc component: Transmission voltages or levels can be
categorized as being UP or BP.
• Unipolar transmission of binary data involves the transmission of only a single non
zero voltage level(either a +ve or –ve voltage for a logic 1 and 0v for a logic 0).
•In bipolar transmission, two nonzero voltages are involved(a +ve voltage for a logic 1
and equal magnitude negative voltage for a logic 0 or vice versa)
Digital Biphase: It is sometimes called the Manchester or diphase, which is a popular
type of line encoding that produces a timing component for clock recovery and does
not cause DC wandering.
•Here equal probability of 1s and 0s are taken, then the average dc voltage is ov, and
there is no wandering .
•A disadvantage of biphase is that it contains no means of error detection
T Carrier Systems
T carriers are used for the transmission of PCM-encoded time division multiplexed
digital signals
T1 Carrier System
Transmission of 24, 64-kbps channels, T1 line speed 1.544 Mbps.
Lengths from about 1 mile to over 50 miles.
Binary eight zero substitution, B8ZS--- (+-0-+000) or (-+0+-000)
T2 Carrier System
Transmission of 94, 64-kbps channels, T2 line speed 6.312 Mbps.
Lengths up to 500 miles.
Binary six zero substitution, B6ZS--- (0-+0+-) or (0+-0-+)
T3 Carrier System
Transmission of 672, 64-kbps channels, T3 line speed 44.736 Mbps.
Binary three zero substitution, B3ZS
T4M Carrier System
Transmission of 4032, 64-kbps channels, T4 line speed 274.176 Mbps.
Lengths up to 500 miles
T5 Carrier System
Transmission of 8064, 64-kbps channels, T5 line speed 560.160 Mbps
European Time-Division Multiplexing
•In Europe, a different version of T carrier lines is used called E lines.
•A high speed digital communications link that enables the transmission of voice, data,
and video signals at a rate of 2.048 Mbps
a) Initially designed for transmission of 30 telephone channels
b) Basis for design: PCM voice digitizing using 64 kbps for each channel.
The E1 frame consists of 32 8-bit channels (timeslots)
32 time slots X
frame
8 bits
time slots
= 256 bits/frame
E1 frames are transmitted at the rate of 8,000 frames/s
256 bits
frame
X 8,000 frames = 2,048 kbps or 2.048 Mbps
second
Statistical Time Division Multiplexing (STDM)
•STDM is designed to make use of the idle time created when terminals are not
using the multiplexed circuit.
•Like regular TDM, STDM uses time slots, but the time slots are not fixed.
Instead, they are used as needed by the different terminals on the multiplexed
circuit.
•Since the source of a data sample is not identified by the time slot it occupies,
additional addressing information must be added to each sample.
•If all terminals try to use the multiplexed circuit intensively, response time delays
can occur. The multiplexer also needs to contain memory to store data in case
more data samples come in than its outgoing circuit capacity can handle.
Beginnin
g flag
Address
field
Control
field
Statistica
l TDM
Subframe
FCS
field
Endin
g Flag
Statistical TDM frame
Address
field
Length
field
Address
field
Control
field
one source per frame
Data field
………
Address
field
Length
field
Multiple sources per frame
Data field
Frame synchronization
•To acquire frame synchronization, a certain amount of overhead must be added
to the transmission
1) Added-Digit Framing:
 Initial frame synchronization depends on the total frame time, the number of
bits per frame, and the period of each bit
synchronization time = 2NT = 2 N2 tb
N = number of bits per frame
T = frame period of N * tb
tb = bit time
For T1 carrier, N= 193, T=125µs, and tb =0.468µs; for 74,498 bits, maximum
average synchronization time is 48.25ms
2) Robbed-Digit Framing
-- When a short frame is used, added-digit framing is inefficient.
-- This occurs with single-channel PCM systems
-- Replace the least significant bit of every nth with a framing bit called robbeddigit framing
-- For n=10, the SQR is impaired by only 1 dB
3) Added-Channel Framing
-- Same as added-digit framing except that digits are added in groups or
words instead of as individual bits
-- The average number of bits to acquire frame synchronization using addedchannel framing is
Number of synchronization bits = N2
N= number of bits per frame
2(2k-1) K= number of bits in the synchronizing
word
4) Statistical Framing
--The second bit is a logic 1 in the central half of the code range and a logic 0 at
the extremes
-- Second digit of a given channel can be used for the framing bit
5) Unique-Line Code Framing
-- Some property of the framing bit is different from the data bits
-- The framing bit is either made higher or lower in amplitude or with different
time duration
-- Either added-digit or added-word framing can be used, or specified data bits
can be used to simultaneously convey information and carry synchronizing
signals
Frequency Division Multiplexing
--With FDM, multiple sources that originally occupied the same frequency
spectrum are each converted to a different frequency band and transmitted
simultaneously over a single transmission medium
--Each narrowband channel is converted to a different location in the total
frequency spectrum
--Channel 1 signals amplitude modulate a 100-kHz carrier in a balanced
modulator, which inherently suppresses the 100-kHz carrier
--The output of the balanced modulator is a double-sideband suppressed carrier
waveform with a bandwidth of 10 kHz
--The double-sideband waveform passes through a band pass filter (BPF) where it
is converted to a single-sideband signal
--The lower sideband is blocked for this example, the output of BPF occupies the
frequency band between 100kHz and 105 kHz.
--The total combined bandwidth is equal to 20 kHz and each channel occupies a
different 6-kHz portion of the total 20-kHz bandwidth.
--The applications for FDM are commercial FM, television broadcasting, highvolume telephone and data communications systems, cable television and data
distribution networks.
Figure 6.3 Frequency-division multiplexing
Note
FDM is an analog multiplexing technique
that combines analog signals.
Figure 6.4 FDM process
Figure 6.5 FDM demultiplexing example
Figure 6.9 Analog hierarchy
AT & T’s FDM Hierarchy
Voice-band
data modem
Channel1
Message Channel: The message channel is the basic building block of the FDM
hierarchy. It was originally intended for analog transmission of voice signals,
although it now includes any transmissions that utilize voce-band frequencies (0
kHz to4 kHz) such as data transmission using voice-band data modems
Basic group: A group is the next higher level in the FDM hierarchy above the basic
message channel and consequently is the first multiplexing step for combining
message channels. Here Twelve 4 kHz voice-band channels occupy a combined
bandwidth of 48 kHz (4 * 12)
Basic supergroup: The next higher level in the FDM hierarchy is the super group,
formed by frequency-division multiplexing five groups containing 12 channels each,
for a combined bandwidth of 240-kHz (5 groups * 48 kHz/group)
Basic mastergroup-- The next higher level of multiplexing, is the master group,
which is formed by frequency-division multiplexing 10 super groups together for a
combined capacity of 600 voice-band message channels occupying a bandwidth of
2.4 MHz (600 channels * 4 kHz/channel)
•Three master groups are frequency-division multiplexed together and placed on a
single microwave or satellite radio channel. The capacity is 1800 VB channels
utilizing a combined bandwidth of 7.2 MHz
(3master groups * 600 channels/master group)
•Master groups can be further multiplexed in master group banks to form jumbo
groups (3600 VB channels), multi jumbo groups (7200 VB channels) and
superjumbo groups (10,800 VB channels).
Wavelength-Division Multiplexing
-- Different wavelengths carry separate signals
-- Multiplex into shared optical fiber
-- Each wavelength like a separate circuit
-- The wavelength spectrum used is in the region of 1300 nm or 1500 nm
-- WDM is a process in which different sources of information (channels) are
propagated down an optical fiber on different wavelengths where the different
wavelengths do not interfere with each other
Dense wave division Multiplexing:
Synchronous Optical Network
•The Synchronous Optical Network (SONET) is a multiplexing system similar to
conventional TDM except SONET was developed to be used with optical fibers
•SONET has some standards